A directional control valve is a valve for switching between a state where fluid flows and a state where the flow is shut off, or for switching the flow direction of the fluid from one port to another port. For example, in a pneumatic circuit for supplying compressed air from an air pressure source to a pneumatically actuated device, a directional control valve is used for switching between a state where compressed air is supplied from the air pressure source to the pneumatically actuated device and a state where supply of compressed air to the pneumatically actuated device is stopped to discharge the compressed air returned from the pneumatically actuated device.
Such a directional control valve has a valve casing in which a valve shaft is accommodated so as to be reciprocable in the axial direction, and a supply port, an output port, and an air discharge port are formed in the valve casing so as to communicate with the valve accommodating hole in which the valve shaft is accommodated so as to be movable in the axial direction. The air supply port is connected to an air pressure source, the output port is connected to a pneumatically actuated device, and the air discharge port is open to the atmospheric air directly or via an air discharge channel. A directional control valve provided with each type of the above-described ports is called a 3-port valve, and a directional control valve provided with one air supply port, two output ports, and two air discharge ports is called a 5-port valve.
The driving methods of the valve shaft include a direct actuation method in which the valve shaft is directly driven by an electromagnet and an indirect actuation method in which the valve shaft is actuated by compressed air. The indirect actuation method includes an external pilot type in which compressed air is supplied to a pilot valve from outside and an internal pilot type in which compressed air is supplied to a pilot valve from inside of the directional control valve. Furthermore, the driving methods of the valve shaft include a mechanical method in which a valve shaft is driven by a cam or a link mechanism and a manual operation method in which a valve shaft is driven manually.
Directional control valves of an indirect actuation type include a single solenoid type in which the valve is driven by one solenoid and a double solenoid type in which the valve is driven by two solenoids. The single solenoid type is a type in which, when electricity supplied to a solenoid is stopped, the valve shaft is returned to the original position, wherein the returning force is applied to the valve shaft by a spring or an air pressure. On the other hand, the double solenoid type is a type in which, even when electricity supplied to a solenoid to which electricity has been supplied is stopped, the valve shaft keeps the position at the time when the electricity is supplied.
As an example of the directional control valve, as described in Japanese Patent Publication No. 2826460, the directional control valve of a spool type having a valve shaft to which a plurality of rubber elastic valve elements each comprising an O ring are attached is used, wherein when mutual communication between ports is to be shut off, the respective elastic valve element is brought into contact with the inner peripheral surface of the valve accommodating hole between the ports. Each of the elastic valve elements is attached to a circular groove formed in the valve shaft. As described in the publication, the type of a directional control valve in which elastic valve elements are attached to a valve shaft has an advantage that sealing performance can be enhanced with low cost in comparison with a metal seal structure in which a valve element made of metal is brought into contact with a valve accommodating hole with a slight gap. On the other hand, there is a limit on the pressure of compressed air for performing switching control of the flow.
That is, the type of a directional control valve in which elastic valve elements are attached to a valve shaft is generally used in a pneumatic circuit to which compressed air with a pressure of about 0.7 to 1 MPa is applied. However, it is found out that, when the directional control valve is used in a pneumatic circuit to which compressed air with a pressure higher than that is supplied, the valve life is shortened due to damage or deformation of the elastic valve elements. As a result of the investigation for the cause thereof, it is found out that, while deformation or damage does not occur in the elastic valve element for switching between a state where an air supply port and an output port are communicating and a state where the communication is shut off even when the directional control valve is used over a long period of time, the elastic valve element for switching between a state where the output port and the air discharge port are communicating and a state where the communication is shut off is deformed or damaged in a comparatively short period of time.
In such a circumstance, as a result of the observation for the switching state of the channel in the directional control valve used in a high-pressure pneumatic circuit of about 2 MPa, it is found out that, when the both ports are communicating, the elastic valve element for switching the state of the output port and the air discharge port between a communication state and a shut-off state is spaced away from the bottom surface of the groove formed in the valve shaft and elastically deformed and extended outwardly in the radial direction. This phenomenon is not found in the elastic valve element for switching the state of the air supply port and the output port between a communication state and a shut-off state. Such phenomenon occurs probably for the reason that, when the output port and the air discharge port are in a communication state, the elastic valve element comes to the position corresponding to the discharge port, therefore, the elastic valve element is blown out by the air which flows into the valve accommodating hole through the output port and then flows toward outside in the radial direction through the air discharge port. When the elastic valve element in a communication state is elastically deformed due to the blowing-out phenomenon such that the radius thereof is extended by the air flowing toward outside and the elastic valve element is moved to attain a communication shut-off state while the above-described state is maintained, an outer peripheral portion of the elastic valve element is sometimes pinched between the inner peripheral edge of the valve hole and the valve shaft. If the blowing-out phenomenon is repeated, the elastic valve element is damaged, thereby shortening the life of the directional control valve and deteriorating the durability thereof. In order to prevent the blowing-out phenomenon, an attempt that the material of the rubber of the elastic valve element is changed to that having an increased hardness is made. However, when the hardness is increased, attachment performance of the elastic valve element to the valve shaft is deteriorated.
An object of the present invention is to provide a directional control valve which can prevent the blowing-out phenomenon of a valve element for switching ports.
Another object of the present invention is to improve the durability of the valve element, thereby providing a directional control valve having a long product life.
Another object of the present invention is to prevent increase in size of a valve casing even when an attachment hole for attaching the valve casing to another member is provided in the valve casing, thereby providing a small size directional control valve.